I started off hardly able to run around the block. Gradually increasing my pace and my distance it was becoming closer to a reality. I got up to being able to maintain the needed pace for the next age bracket up over a 1/2 marathon distance and was improving. Then my Achilles went poof followed by my knee. Then they changed the standard to make it even tougher. Now I'm content to put in 15 - 20 miles a week at a much more leisurely pace and feel good about life.
At least setting the goal got me off the couch to begin with but the marathon isn't happening.